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You spend hours trying to achieve the perfect paint job only to face a re-do and an irate customer. But don’t go blaming your paint. The culprit is likely your spray gun not being properly adjusted or as clean as you think.
MISUNDERSTOOD, ABUSED, NEGLECTED AND DIRTY.
These four words describe the most important tool in your paint department: the atomizing spray gun. Lots of work goes into great paint jobs, but no single operation is more important than a smooth, glossy topcoat. But that won’t happen regularly unless your spray gun is clean, properly adjusted and suitably setup. And it doesn’t happen as regularly as it should in many shops out there — maybe even yours. This is why I interviewed several leading spray gun manufacturers and asked them each for a universal painting tip — something that would apply to any auto painter regardless of spray gun brand. I also asked for a particular feature of their product line that made the painter’s job easier. They all had good advice.
When I spoke to experts at the spray gun companies, I posed my first question like this: "Imagine I’m about to start a career as an auto painter tomorrow morning. What’s the most important thing I should know?" I got great advice from everyone, and in logical order, from the beginning to the end of the paint job, here it is …
Tip No. 1: Prepare — or Prepare to Fail
According to the president of one spray gun manufacturer, a great paint job is almost impossible without surface preparation. No spray gun or paint brand will compensate for a poor foundation. Unless the surface is chemically clean, perfectly smooth and composed of up-to-date primers, putties and sealers, the final result will suffer. (I know, I know … it seems like every article you’ve ever read about painting starts this same way. And certainly every paint manufacturer’s school starts with a discussion about solid preparation. That’s because it’s so important!)
So if everyone already knows how important it is, why do experts and trainers still harp on it? Because painters are paid on labor time, and the temptation to skip steps is hard to resist. The old saying, "There’s always time to do it over, but never time to do it right the first time" applies to a certain number of automotive painters every year. Cutting corners, skipping steps and "saving time" sounds great until the job blows up.
When I was a jobber, the worst customer to confront was one who had six or eight successful paint jobs while skipping some preparation steps. And when his oversights finally caught up to him and the next paint job failed and had to be re-done, it was always the paint’s fault. "If there was going to be a problem," he’d shout, "why didn’t it happen to the first several jobs?" One likely answer was "luck."
Skipping a single step may not affect the durability of the paint job very much. If all the other conditions are in order, automotive undercoats and paints are pretty forgiving. But skip a couple of steps, get casual about your sanding or solvent wiping and look out.
Great paint jobs always start with great preparation. Commit to getting it right the first time — your customers will thank you for it.
Tip No. 2: Quality Air = Quality Job
Another spray gun equipment company president says the most important elements in any painter’s job are high-quality compressed air and a good spray gun. He rightly makes the point that no spray gun or paint resin will perform well if there isn’t enough clean, dry air to supply the operation. How much compressed air is enough? How much air does your compressor produce? Here are some helpful guidelines.
- If your air compressor has an electric motor of at least 5 HP, you should displace approximately 4 cubic feet per minute (CFM) of air for each unit of horsepower. So a 10 HP compressor should displace somewhere around 40 CFM (4 CFM x 10 HP = 40 CFM).
- Smaller compressors with less than 5 HP electric motors generally don’t do quite as well. You should figure their approximate displacement at around 3 to 3.5 CFM per each unit of horsepower. So a 1.5 HP air compressor should displace about 4.5 to 5 CFM (1.5 HP x 3-3.5 CFM = 4.5-5 CFM).
How much air an air compressor actually displaces is a mathematical formula. Multiply the bore of the cylinder by the length of the stroke by the RPM of the pump to find exactly how much air is displaced by the pump each minute. How much air is actually delivered and available at the outlet on the tank is less than the air displaced by the pistons.
One way to compare air compressors when shopping for a new one is to calculate the difference between displaced air volume and delivered air volume. If an air compressor displaced 20 CFM and delivered 16 CFM at 90 PSI, it would be 80 percent efficient (16 divided by 20 = 80 percent). A competitive compressor might displace 20 CFM, yet deliver only 14 CFM for an efficiency of 70 percent (14 divided by 20 = 70 percent). Note that more air pressure means less air volume.
In this example, if the compressor’s output were measured at 40 PSI instead of 90 PSI, the air volume would be higher. This is one reason you see cheapo air compressors in the Sunday newspaper supplements rated at 40 PSI; it shows a higher delivered volume. Make sure you match the spray gun to the air supply available.
With the industry-wide adoption of high-volume low-pressure (HVLP) spray guns, many paint shops don’t have enough air volume to properly operate their spray equipment. And without sufficient "high volume," the guns can’t atomize the paint properly. Poor coverage, runs and orange peel are the results.
You say your brand of gun needs 15 CFM to operate properly and your shop has a 10 HP compressor — or roughly 32 CFM of delivered air? No problem, right? Right … as long as no one else in the entire shop is using air at the same time. Add two dual-action sanders at 8-10 CFM each, an air grinder at 20 CFM consumption or another spray gun, and all of a sudden, your spray gun is starved for air. Consider adding another compressor rather than buying a bigger replacement pump. With two air compressors, the shop can still do some work if one breaks down.
Clean compressed air means the water vapor, lubricating oil vapor and rusty scale are removed before the air goes through the spray gun. Moisture traps come in several styles and designs — from metal baffles to toilet paper rolls to desiccant beads. Whichever style you choose, make sure to service it regularly. The best design in the world won’t work well if it’s never cleaned or drained.
Tip No. 3: Follow Instructions
There are probably 25 variables each time you paint: distance, PSI, CFM, proper material mix, gun orientation, tip size, etc. And if only two or three variables are off, the paint job will suffer. One spray gun company representative reminds us that HVLP guns are precision devices. If you ignore the manufacturer’s specific recommendations for air pressure and tip size, your results will suffer.
Some areas of the country forbid spray guns with more than 10 PSI at the air cap. Some paint gun manufacturers mandate exact PSI requirements at the air cap for different coatings. The reading on the regulator’s pressure gauge, far away on the booth wall, isn’t accurate enough. The surest way to know you have the correct air pressure at the air cap is to purchase an air cap test gauge. Available from virtually every spray gun manufacturer, these devices cost $50-$60, basically the cost of a regular air cap plus the cost of an air pressure gauge. Simply remove the regular air cap and screw on the test cap. Pull the trigger on the gun and read how much air pressure is escaping. This is an operation that should be performed every couple of months to check that the air supply is consistent. If the regulator on the booth wall reads 50 PSI to achieve the desired 10 PSI at the air cap, your painter can work off the wall gauge confidently for a few months.
Equally important is to have the correct fluid tip diameter. Many, many painters are spraying the new high-solids, low-solvent coatings with fluid tips that are too large. But wait, if labor time is so important, why wouldn’t big diameter fluid holes be faster? Large center holes allow lots of paint to pass through them. Lots of paint resin requires lots of air to atomize the droplets small enough to flow out. Lots of air (volume and pressure) means lots of overspray blowing around. Lots of overspray means very poor transfer efficiency. Hard as it is to believe, smaller diameter fluid nozzles can provide better paint jobs, faster.
Equipping your spray gun with the correct fluid tip and fluid needle (matched size set) will ensure you’re getting the maximum amount on the car as well as even flow. Too large a fluid hole in the gun can result in orange peel. Both your spray gun manufacturer and your paint manufacturer have very specific recommendations about fluid tips and air caps; follow them exactly for best results.
Tip No. 4: Take a Test Run
When I started painting back in the Stone Age, we used siphon-feed spray guns at high air pressure (65-70 PSI). You got a satisfying "kick" from the gun when you pulled the trigger, and the air made a healthy hissing noise as it escaped. However, all that escaping air reduced the amount of paint that actually hit the surface to about one-third of what was in the cup. The other two-thirds flew up into the booth exhaust and disappeared outside.
New gravity-feed HVLP guns apply two-thirds of the paint to the vehicle, with only one-third escaping into the atmosphere. However, there’s very little "kick" to the HVLP guns, but don’t let that fool you. You can’t successfully judge any atomization air pressure by sound.
I asked my friend — who’s in charge of new product development and corporate training for a major spray gun company — what suggestion he’d offer a new painter. His reply was to always shoot test panels. Not just for the correctness of the color, but for the performance of the spray gun as well.
On clean white paper, shoot a spray pattern at the exact same distance you’ll use when you spray the car. Use a ruler to make sure. I asked him whether the painter should turn the horns of the air cap to face north and south, rather than the normal east to west orientation when shooting a test pattern. The theory here is that defects in the spray pattern are more easily identified if the pattern is parallel, not perpendicular, to the ground. But my friend wasn’t convinced this was necessary. He suggested producing a large, wet line from top to bottom and edge to edge with no holes or gaps. Look for the droplet size to be uniform and small enough to cover the white test paper thoroughly.
Doing this simple test each time will identify a gun that’s starved for air, has too much air pressure, and is obstructed or misadjusted. If the spray test pattern doesn’t look good, the paint job will definitely suffer. Many cases of painters blaming the gun or the paint can be traced back to dirty, starved or poorly adjusted spray guns.
Tip No. 5: Create a Set Pattern
Another spray gun manufacturer representative I spoke with suggested the most important thing for a painter is uniform pattern overlap when spraying. For years, the industry standard was a 50 percent overlap of the next gun pass to the previous pass. But a larger overlap, like 75 percent, will make for coverage in less coats, a more even application and better durability through consistent mil build. EPA research also suggests that measured, consistent gun passes will reduce or eliminate stripes and mottling as well.
How can something that’s described as an "art" like spray gun handling, respond so well to exact machine-like techniques? The vehicle was painted originally under very controlled conditions — in fact, most coatings were actually applied by a machine. The robot painter makes each pass identical to the last pass both in distance to the surface and overlap. Factory technicians doing their best to act like machines did those few OEM coatings that were hand sprayed.
Gun techniques — the actual waving of the gun around the vehicle — are the skills that separate the great painters from the average ones. Adjusting the gun each time, testing the pattern, keeping a consistent distance, maintaining the exact overlap and crouching down or standing on a stool to remain perfectly parallel to the surface make for great paint jobs. They even take less time. If you know your overlap and spray distance are consistent, you know your coating thickness is, too. And you’ll reach hiding and minimum mil build quicker than if you casually wave the gun at the panel again and again.
Make that Money Coat Count!
What are the necessary ingredients for a flawless finish? Prepare the substrate carefully, drain all the moisture traps regularly, adjust the many variables each time you spray, follow the gun and paint manufacturers’ recommendations religiously and take the time to get trained on how to get the most from your brand of spray gun.
The very last pass of the gun is called the "money coat" for a reason. This is the only part of a long and detailed process the painter undertook to complete the job that the customers actually see. Make sure they’re impressed.
Writer Mark Clark, owner of Professional PBE Systems in Waterloo, Iowa, is a well-known industry speaker and consultant. He’s been a contributing editor to BodyShop Business since 1988.
Even a Know-it-All Doesn’t Know Everything
training. Just because you’ve been painting for years doesn’t mean you know it all. The prima donnas who never read the directions and already know everything will work harder to achieve a flawless finish. Granted, if you’ve been painting cars for many years, you can overcome a lot of problems with your skills. A much better approach, however,
is to identify and eliminate the problems before they occur.
Most paint gun manufacturers offer training classes from one to three days long.
Commit to spending a day or two learning how to get the best results from your spray gun.
Get the Tools to Do the Job Right
• Fluid tips (also called fluid nozzles) are the part of the gun under the screw-off air cap. Each brand’s design has hours of engineering in its creation and careful machining in its execution. One of the reasons quality spray guns are expensive is the careful hand or CNC (computer numerical control) machining that makes each fluid nozzle a precision part. Cheap, throwaway guns have their fluid tips sand cast, not drilled and machined. Cast parts are far less durable and precise.
One brand has a fluid tip with a unique flange, another’s tip design is concave, still another concentric and others have 12 points. One brand sells theirs in two pieces so you only have to replace the center, which is the part that wears. In each case, the people I spoke with are sure theirs is the best solution to a great fluid nozzle.
How will you tell? Test spray two or three jobs with their gun. It doesn’t matter to you if the design is flared, flanged, hexagonal or made from unobtainium; the proof is in the finish. Use your own paint brand in your own booth to get a true test. Try several brands but follow their suggestion for tip size, air cap and air pressure.
• Atomization is the heart of any spray gun. The goal is to get the smoothest paint job while still maintaining high (65 percent) transfer efficiency. If you make the paint droplets or particles too small, the job will lay flat but the tiny droplets and fine overspray will drift off target.
Several gun manufacturers actually count the droplets within a square inch for each aircap and fluid nozzle combination. Depending on the brand, there can be more than 1,000 different cap/nozzle combinations. Some would produce droplets too large for auto paint but just the right amount for rubber undercoating or a truck bedliner. Some would produce droplets so small it would be like spraying baby powder. Some guns are designed to atomize the paint in two steps, once behind the nozzle and again in front of the fluid outlet hole.
All great operating spray guns have a smooth and uniform column of escaping air. This feature prevents spikes and bursts that can affect the spray pattern. Remember also that smooth paint jobs are a result of slow solvents as well as great air atomization. Whichever spray gun you try, slow your solvent down for the best results.
Other features run the gamut from quick-release air caps to built-in pressure gauges. Some gun manufacturers actually test spray every spray gun before it leaves the factory. Some brands have invested hundreds of thousands of dollars in the absolute latest CNC lathes to keep their quality perfect. Some brands tout their low air consumption (6-8 CFM required), and some brands are particularly proud of the high-grade stainless steel and brass they use in manufacturing their gun. In every case, the proof is in the spraying. Make sure you try before you buy to get the best gun for your situation.